Distribution of ataxin-7 in normal human brain and retina.

Spinocerebellar ataxia 7 (SCA7) is a neurodegenerative disease caused by the expansion of a CAG repeat encoding a polyglutamine tract in the protein ataxin-7. We developed antibodies directed against two different parts of the ataxin-7 protein and studied its distribution in brain and peripheral tissue from healthy subjects. Normal ataxin-7 was widely expressed in brain, retina and peripheral tissues, including striated muscle, testis and thyroid gland. In the brain, expression of ataxin-7 was not limited to areas in which neurones degenerate, and the level of expression was not related to the severity of neuronal loss. Immunoreactivity was low in some vulnerable populations of neurones, such as Purkinje cells. In neurones, ataxin-7 was found in the cell bodies and in processes. Nuclear labelling was also observed in some neurones, but was not related to the distribution of intranuclear inclusions observed in an SCA7 patient. In this patient, the proportion of neurones with nuclear labelling was higher, on average, in regions with neuronal loss. Double immunolabelling coupled with confocal microscopy showed that ataxin-7 colocalized with BiP, a marker of the endoplasmic reticulum, but not with markers of mitochondria or the trans-Golgi network.